Here is an example of multiple transactions in the same block, all spending from the same output. I created these using the Electrum wallet, telling Electrum to spend a little bit of the BTC from the same 1AD9xV6aeQMPMzfv5Nzk8yNDgtyV3LaUkB address each time

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Adding my own answer after doing some research: An address and an output are different things. A transaction may have many inputs and many outputs. Here is a good visualization from bitcoin.org:

A double spend occurs when the same BTC is used to pay two different people. This will never happen on the blockchain because the Bitcoin network guarantees a consistent ledger. But, what about the unconfirmed transactions that live in the miners' mempools?

Some inconsistencies in the mempool may not manifest until they are added or not-added to the blockchain, others we can flag as invalid right away e.g. amount exceeds available funds etc; If unconfirmed transactions were not checked for validity, an attacker could flood the mempool with bogus transactions and cripple the network.

A submitted transaction that has the exact same inputs and outputs as another unconfirmed transaction in the mempool is rejected by the Bitcoin network (even if it was a valid financial transaction). For example, I could send 1 BTC from A to B, and while the transaction is unconfirmed, send another 2 BTC from A to B. As long as A has 3 BTC to cover the transfers, these transactions are consistent, but conservatively rejected by miners.

You can see this yourself with the Electrum wallet using the "Send From" option. Normally, Electrum picks valid inputs for you, but you can override this. Create a transaction specifying the from and to addresses. Then, create another transaction with same from and to (but the amount can differ). If the first transaction is still pending, you'll get the following error: